Perimeter intrusion alarm



e 0R 3mm e 02?), T If' April 8, 1.969 A. NELKTN ET AL 3,438,021

PERIMETER INTRUSION ALARM Filed July 26, 1965 sheet ef 5 INTERROGATING ALARM MON|TOR|NG T SIGNAL INDICATING UNIT r GENERATOR UNIT E ,el 3L PROTECTED REGION 5h. PREAMPL'FIER l Isa L I SECTION IOT UNDISTURBED GROUND III BACKFILL April 8, 1969 A, NELKIN ET AL 3,438,021

PERIMETER INTRUSION ALARM Filed July 2e, 1965 sheet Z TEE al@ FIG.' 4' 3?` FIG. 5. Q43

yZ/g--Z/ (7 //ii-i I 7T 7T 95 TRANSDUCER R M F|G.6. DIFFERENTIAL SECTION OFHOsE =`Ch T-Ch EI sIGNAL= E DIRECTION DIRECTION OF ACCEL. OF ACCEL. NEUTRAL F'G- 90' NEUTRAL F|G.9b.

' PLANE IO IOI '0l/717 9| 9| lOl April 8, 1969 A. NELKIN ET AL PERIMETEH INTRUS ION ALARM Sheet Filed July 26. 1965 A. NELKIN ET AL- PERIMETER INTRUsIoN ALARM April s, 1969 Sheet4 :j of 5 Filed July 2G, 1.965

United States U.S. Cl. 340-261 9 Claims ABSTRACT OF. THE DISCLOSURE This invention relates to a perimeter intrusion alarm system which includes a flexible hose member containing a iluid capable of transmission of energy impulses in response toV pressure applied to the external portion of the ilexible tube. The pressure impulses are detected by a transducer means which converts this mechanical energy into an electrical energy manifestation and indicates an intrusion over the llexible detector hose means. In addition, the system provides a piezo-electric crystal member for sensing said intrusion and also means for interrogating said system by means of providing an interrogating energy pulse into said detection system.

This invention relates to security signal apparatus and has particular relationship to apparatus for detecting the penetration of a boundary of a predetermined region. A typical purpose of such apparatus is to sense that persons or objects are trespassing across a perimeter or barrier of a restricted area or region and are intruding into the area or region. Typical applications of this apparatus include the protection from sabotage of earthen bank oil reservoirs in such localities as Venezuela, the detection of iniltrators in South Vietnam, the security of defense installations, the White House, and the like. Such applications require a perimeter detector which has high sensitivity to sounds or pressures in the immediate vicinity of the protected perimeter and low sensitivity to such sounds or pressures away from this immediate vicinity.

It is broadly an object of this invention to provide security signal apparatus which shall meet these conditions.

Security signal apparatus includes a detector disposed above the perimeter barrier or boundary protected and alarm-signaling apparatus for indicating intrusion through or over the boundary. Security signal apparatus capable of meeting the above-stated object should have the following properties.

(1) It should be capable of detecting a muied attempt to cross the protected perimeter; for example, a person crawling, walking or attempting in any way to cross over a specific strip of ground at the perimeter or the movement of an inanimate object either automatically or by remote control over this ground.

(2) It should include a detector which is completely concealed.

(3) The detector should trip an alarm in the alarm signaling device when an attempt is made to destroy or disable the detector or any of its connections to the alarm signaling device.

(4) The detector should require a minimum of maintenance.

(5) The apparatus should be such that detection over a large area with a minimum number of detectors and minimal signaling apparatus is feasible.

(6) The apparatus should be immune to disturbances outside of the strip over which the detection takes place but should at the same time have high sensitivity in this strip.

atent Patented Apr. 8, 1969 nice `(7) The apparatus should be of the fail-safe type and should indicate, by an alarm, failure of any detectors or of any other parts of the apparatus.

In accordance with the teachings of the prior art security signaling apparatus with electrically or electromagnetically operating detectors have been proposed. Typically, such apparatus has included detectors of the photoelectric type and of the microwave type and single transducer seismic detectors. This apparatus has not satisfactorily met the above-described conditions.

It is an object of this invention to overcome the deiiciencies of the prior art apparatus and to provide security signaling apparatus which shall meet the abovelisted conditions.

In accordance with this invention, security signaling apparatus in which the detector is of the electro-acoustical type is provided. The principal detecting component of this apparatus is a fluid-filled pressure conductor or tube terminating in a transducer which responds to pressure applied to it by an electrical manifestation; for example, by producing an electrical potential. The material of which the fluid-filled tubes are made is highly compliant so that a small impulse or a small pressure applied to the walls of the tubes causes a pressure wave to ow through the tubes to the transducers. In accordance with the specific aspects of this invention the transducer is a piezo-electric crystal which produces an electrical potential across output terminals when pressure is applied to it. Typical crystals are composed of lead zirconate-titanate, barium titanate, or quartz or related materials. Within the broader aspects of this invention, transducers or other types may be used. For example, the iluid-lled tube may terminate in a linear-variable-differential transformer, usually referred to as an LVDT,

or in a highly sensitive capacitor whose plates are af,

fected by the iluid. The unit including the fluid-filled tube and the transducer operates acoustically. The sound or mechanical pressure of the steps of an approaching intruder causes pressure impulses to flow along the iluid in the tube. These impulses produce an electrical potential which may serve to actuate an alarm signal. This detector is capable of detecting the very faint muffled sounds or pressure produced by quiet attempted penetration of a boundary as of a person crawling near or across a boundary. The frequencies of these sounds may be very low, or infrasonic, or the impulses may be aperiodic.

In accordance with an aspect of this invention, the detector unit is provided with facilities for continuously checking its integrity or its capability of operating. For this purpose a second transducer similar to the one mentioned above is connected to the opposite end of the fluidlled tube in communication with the fluid. On this latter transducer, an interrogating signal potential of low intensity is impressed. Where subjected to such a potential the transducer impresses pressure impulses on the uid and these are transmitted along the lluid to the transducer at the other end which is to pick up the noise of an intrusion. The latter responds to the interrogating signal to produce a manifestation indicating that it is operational which continuously informs an attendant that the detector apparatus is in operating condition.

In accordance with an aspect of this invention, sensitivity to local sounds or other disturbances and insensitivity to remote sounds or disturbances is achieved by providing a pair of flued-lled tube detector components at each section of the boundary or perimeter to be detected. The assembly including the fluid-lled tubes and the transducers at the end of the tuibe is referred to herein as a detector unit. The tubes are spaced a relatively short distance typically of the order of 2 to 5 feet. Preferably the tubes may be displaced horizontally but they may also be displaced vertically or along a plane at an angle'other than 90 or 180 to the surface. The intruder pick-up transducers at one end of each of the tubes are connected with reversed polarity, or opposite phasing to the alarm indicator` The connection is such that `pressure disturbances transmitted to the two tubes simultaneously with substantially equal intensity produce no alarm indication. An alarm indication is only produced when the pressure disturbances transmitted along the lluid of the two tubes produce a net disturbance in the transducers, that is, the effect of the pressure disturbances impinging on each of the transducers is to cause one transducer to produce an electrical signal greater or smaller than the other.

.Typically the interrogating transducers at the other ends of the tubes are also connected with opposite phasing. A common small interrogating signal is impressed on these transducers which causes pressure impulses to be transmitted along the respective fluids. These pressure signals produce a small net output from the pick-up transducers which serves to monitor the integrity of the complete fluid-transducer assembly.

'In accordance with this invention, the detector unit is under the surface of the perimeter being protected. -In the usual situation when the perimeter is outside of a building or buildings, the detector units including the tubes and the transducers and any preamplitiers are buried in the ground and the output cables from the preampliers are underground. The Idetecting unit is thus completely sealed from intruders. Typically, the detector units may be buried about 6 inches underground with the ground arorund the detector units tightly packed so as to transmit the pressure impressed on the surface. In areas in which the temperature may be relatively low, the iluid within the resilient or pliant tubes is of the type that has a low freezing temperature. In accordance with a specific aspect of this invention, the lluid may be a solution of glycerin or its derivatives (for example, Prestone liquid) and water.

For a perimeter of substantial length a number of detectors are used. It has been found that for a detector having a length of about four hundred feet, the response to a disturbance at the end of the detector remote from the intrusion responsive transducer is only about 4 db below the response near the transducers. Each of the detector units for long perimeters may then be between about 300 and 500 ft. in length. The detector units should -be so spaced around the periphery that there are no gaps between detectors through which undetected intrusion may occur. In certain situations there may be advantages in overlapping the detector units.

For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following :description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic view showing a preferred embodiment of this invention;

FIG. 2 is a View in perspective showing a detector unit in accordance with this invention including the Huidlled tubes and their associated transducers;

FIG. 3 is a view in perspective showing the manner in which the detector unit is buried in the practice of this invention;

FIG. 4 is a view in section showing the connection of the uid of the detector unit to the transducers;

FIG.5 is a view in section taken alonlg V-V of FIG. 4;

FIG, 6 is a schematic of an equivalent electrical circuit corresponding to the detector unit for situations in which the frequency of the impulses is of such magnitude that the mass of the illuid is appreciable;

FIG. 7 is a corresponding equivalent circuit where the frequency of the impulses transmitted along the fluid is very low;

FIG. 8a shows the reaction of a transducer diaphragm due to an increase in Huid pressure in one of the tubes as in detection ef intrusion.

FIGS. 8bv and 8c show the deilections and the reaction on the transducer elements for accelerations of the entire detecting assembly, due to earthquakes or other largescale vibrations.

FIGS. 9a and 9b are views, enlarged and in section, of the transducer elements showing the relationship of the diaphragm and the piezo-electric crystal;

FIGS. 10 and l1 are schematic showing the manner in which the transducer elements may be electrically balanced;

FIG. 12 is an oscillogram showing the response of the apparatus according to this invention to movement parallel to a buried detector unit in accordance with this invention;

FIG. 12a shows the direction and'character of movement which produced the oscillogram shown in FIG. 12;

FIG. 13 is an oscillogram produced when a buried detector unit in accordance with this invention is crossed;

FIG. 13a shows the movement w-hich resulted in th oscillogram of FIG. 13; v

FIG. l4 is a schematic showing the preamplifier and the amplier in the alarm indicating apparatus; and

FIG. 15 is a schematic similar to FIG. 14, but showing the actual components in apparatus used in the practice of this invention which has been found to operate satisfactorily.

FIG. l5 is self-explanatory and is presented here not with any intention of in anyway limiting the scope of this invention but for the purpose of aiding those skilled in the art in practicing this invention.

The apparatus shown in the drawings includes a Detector Unit, a Preamplier, an Alarm Indicating Unit, an Interrogating Signal Generator and a Monitoring Unit. This apparatus is divided into these components for the purpose of explaining this invention; in the actual apparatus according to this invention certain of the above-listed components may be physically combined. In providing security apparatus for any Protected Region, a plurality of the Detector Units each including a Preamplier are disposed around the perimeter of the region. The extent and disposition of the Detector Units depends in each case on the extent of the boundary which can be reasonably identitled. In the practice of this invention, an Alarm Indicating Unit and a Monitoring Unit are connected to respond to, and a Signal Generator is connected to actuate, a plurality of Detector Units. The number of units with which an Alarm Indicating Unit, a Monitoring Unit and an Interrogating Signal Generator are connected to cooperate is determined by the extent to which an attendant may be taxed without becoming so fatigued as to operate ineffectively. In the practice of this invention, the Monitoring Unit and the Alarm Indicating Unit may be included in a single console with adequate light or other indicators to enable an attendant to monitor the security of the protected region. The interrogating signal may be an attenuated potential derived from a commercial alternating-current supply.

In the use of this apparatus one Vor more attendants is positioned to observe the panel of the console and is provided with communicating equipment to take prompt action in the event of an alarm. Any actuation of a Detector Unit produces continuous signals and alarms and in the usual practice of this invention the alarms continue to be energized until corrective action is taken.

The Detector Unit includes a pair of fluid-filled tubes 31 and 33. The tubes 31 and 33 are composed of compliant material such as rubber and the iluid 35 may be a'liquid such as water or water and glycerin or other freezing temperature depressant. l

A Detector Unit made for use in the practice of this invention is shown in FIGS. l, 2 and 3. At each end, each tube 31 and 33 is connected to a metallic union 37 and 39, and 41 and 43 respectively. Each union 37, 39, 41, 43 is provided with a manual valve 45 which is closed during shipment or repair of the light indicators. At one end, the

unions 37 and 41 connect through a T-junction 57 to a container 51 within which the tubes 31 and 33 communicate with the intrusion pick-up transducers 53 and 55 as illustrated in FIG. 4. The remaining arm of each T-junction 57 is connected to a common overflow reservoir 71 through a relie-f valve 73. At the other end eachunion 39 and 43 is connected through a right angle pipe 58 to a container 61 within which they communicate with the interrogating transducers 63 and 65.

The contianers 51 and 61 are similar and FIGS. 4 and 5 only illustrate container 51. The container 51 illustrated in FIGS. 4 and 5 at each end of the Detector Unit is composed of abutting sections 75 and 77 which define chambers 79 and 81 between them. Each chamber includes a transducer 53 and 55. The container 61 is similar and contains transducers 63 and 65. Each chamber 79 and 81 is separated into two parts by the diaphragm 83 of the transducer which it contains. One part of each chamber 79 and 81 contains the fluid 35; the other part contains air. The diaphragm 83 `forms a fluid-tight seal between the parts. Each transducer 53, 55, 63, 65 includes in addition to the diaphragm 83 a piezoelectric ceramic disk 91. The disk 91 is secured centrally to the diaphragm 83 on the side of the air section of each chamber 79, 81. The fluid 35 in the tubes 31 and 33 exerts a pressure against the diaphragm 83 flexing the diaphragm 83 and the disk 91 secured to it. The flexing may be caused by an impulse moving towards the diaphragm 83 or away from it and its direction depends on the direction ofthe pressure impulse.

The flexing causes the disk to produce an electrical potential whose polarity depends on the direction of flexing. This potential is derived through a conductor 93 secured to the disk 91 and a terminal in section 77 of the container 51 or 61. A pressure impulse ltransmitted through the fluid 35 is reflected from the disk 91 on which it impinges producing a stress of opposite polarity so that the disk may be eXed from a position in which it is dished orjconcave as viewed from the fluid to a position in which it is dished convex towards the fluid and potentials of opposite polarity may be produced.

Conversely a potential impressed on the ceramic disk 91 of a transducer causes the transducer to flex. Such a potential is impressed on the interrogating transducers 63 and 65 causing them `to flex and produce impulses on the fluid 35.

The relative thicknesses of the piezo-ceramic disk 91 and o-f the diaphragm S3 of each transducer 53, 55, 63, 65 are such that during flexing the neutral plane of the transducer is in the diaphragm 83. This relationship is illustrated in FIGS. 9a and 9b. FIG. 9a shows the situation which arises when the fluid 35 causes the transducer to be concaved as viewed from the direction of the fluid. In this case both the diaphragm 83 and the ceramic disk 91 are bent. Both the ceramic disk 91 and the portion 101 of the diaphragm 83 on the side of the neutral plane or the ceramic disk 91 are in tension, this tension having a direction parallel to the plane of the disks 91, while the part 103 of the diaphragm on the opposite side of the neutral plane is in compression also parallel to the plane of the disk. For circular disks this tension and compression are radial. Since the whole ceramic disk is in compression, the maximum electrical potential is derived by the flexing of the ceramic disk. A corresponding situation occurs when the transducer is deflected so that it is convexed towards the uid. In this case, the ceramic disk 91 as a whole is in compression and the maximum electrical eflect is again achieved.

In the practiceof this invention the fluid in the reservoir 71 is at such a height as to provide a head relative to the adjacent transducers 53 and 55 such that in the stand-by condition of the apparatus, the disks 91 of the transducers 53 and 55 are subjected to slight tensional stress.

In the practice of this invention the disks 91 at each end are connected bucking, Vthat is, so that substantially 6 the same stress impressed on the disks at each end would produce a zero potential. The connection is illustrated in FIGS. 8a, Sb and 8c. In FIG. 8a, the relationship of the disks when subjected to tensional stress in one of the tubes is shown. `In this case a net signal is produced at output terms 95.

In FIGS. 8b and 8c the etect of equal stresses in the respective opposite directions are shown. In all cases the output is zero when the stresses are alike. Only when the stresses are diierent of the two plates either in polarity or in magnitude net potential appears at the output as in FIG. 8a.

For the purpose of checking the operation of the intrusion pickup transducers 53 and 55 units, an interrogating signal is impressed on the interrogator transducers at the end remote from the reservoir 71. This signal may be of the alternating current type and typically may be a 60- cycle potential derived from an available commercial source. The interrogator signal is impressed in common on the two transducers 63 and 65. The effect of the interrogator signal potential on one transducer is to produce a deflection of one polarity and the effect on the other is to produce a deflection of the opposite polarity so that pulses of opposite polarity are transmitted through the iluid to the intrusion-pickup transducers 53 and 55. The pickup transducers then produce a signal potential at their common output.

The transducers 53 and 55 are shown in FIGS. 8a through 8c as connected respectively bucking in parallel. They may also be connected bucking in series. A similar arrangement is made `with respect to transducers 63 and 65.

It is important that the transducers 53 and 55 and 63 and 65 respectively at each end of the fluid-'filled tubes 31 and 33 be appropriately balanced. The balancing may be affected by a variable capacitor 102 connected across one of the transducers 53 and 63 of each pair 53 and 5S and 63 and 65, respectively (FIGS. 10 and 11), the 'balancing may be omitted if the transducers are selected in matched pairs as part of the manufacturing process.

Referring to FIG. 14, the Preamplier includes fieldeffect transistors F1 and F2 connected in cascade, a transistor Q3 and an integrated circuit or monolithic unit MO. The common input from transducers 53 and 55 is impressed on the base of F1 through a circuit including diodes D1 and D2 which protect the transistor from overvoltage. The transistor F2 compensates for the effects of temperature changes in transistor F1, and serves as a constant-current l'oad for F1. The output of transducers 53 and is composite including the response to the interrogating signal impressed on transducers 63 and 65 by the Interrogating Signal Generator. The interrogating signal is typically a -cycle signal den'ved from a commerical alternating supply. The output of transistor F1 is impressed on the base of transistor Q3, the output of which is supplied to the monolithic element MO. The diodes D5 and D6 protect the capacitor C1 which serves to suppress high-frequency distrubances. The output of the Preamplifier is derived from the output conductor LO of the unit MO.

The output of the Preamplifier is supplied to the Alarm Indicating Unit. This Unit includes the transistors Q1, Q2, Q4, Q5 and Q6. The output from the Preamplifier is impressed on the base of the transistor Q1 through capacitors C2 and C3, C2 being of high capacity. The output of the Preamplier is also impressed on the base of transistor Q5 through filter FI. The filter FI suppresses the 60-cycle interrogating signal so that the potential irnpres'sed on transistor Q5 is only the potential derivable from detection of intrusion. In the absence of intrusion only bias derivable from the resistor R1 is impressed on Qs.

The output-of Q1, which is principally the 60-cycle interrogating signal is impressed on the base of Q2 through resistor R2 and capacitors C4 and C5. Diodes 7 D3 and D4 are connected oppositely across the input to transistor Q2. These diodes maintain the alternating potentials impressed on the base of Q2 no greater than a predetermined level. Typically the potential impressed on Q2 in the usual practice of this invention is limited and has a peak-to-peak amplitude of about l volt by reason of the operation of the diodes D3 and D4. The output of Q2 is derivable through capacitors C7 and C8 and resistor R4. The output of Q2 is also impressed through capacitors C7 and C8, resistor R4 and capacitor C9 on the base of transistor Q6. The output of Q6 is impressed through capacitor C10, rectified by D20 and D21, and applied through resistor R to the base of Q4.Q4 is connected to supply the coil of a relay R. In the standby condition of the apparatus Q4 is conducting and relay R is actuated so that its contact Ra is closed and its back contact Rb is open. The contacts Ra and Rb are connected to' th Alarm Indicator. With Ra closed, the Alarm Indicator indicates only that the Detector Unit is in proper operating condition.

The alarm Indicating Unit includes a discriminator DR which is formed of a plurality of Zener diodes Z1 through Z5`and Z6 through Z10 connected in lseries pairs respectively across supply conductors L1 and L2, Z6, Z2 :and Z7, Z3 and Z8, Z4 and Z9, and Z5 and Z10 are respectively connected in series. The pairs of diodes breakdown for different and progressively increasing voltages. A selector switch CW is provided for selecting any pair of series connected diodes Z1 through Z5 and Z6 through Z10. The signal from the output of Q2 is impressed through capacitors C7 and C8 and resistor R4 on the discriminator DR through the movable arm 104 of selector switch CW.

The selected diodes are normally non-conducting. When a voltage higher than the breakdown voltage of the -selected diodes is impressed on the discriminator DR one diode of the selected pair breaks down for a portion of one half wave of one polarity of this voltage and the other diode of the pair breaks down for -a portion of the other half wave. As shown in FIG. 14, the diodes Z3 and Z8 are connected to co-ntrol the discriminator DR, typically these diodes break down for 8.2 volts. If the voltage during the negative half period of the interrogating potential impressed is of such magnitude that the voltage between the upper potential conductor L1 and the movable arm 104 of the switch CW exceeds 8.2 volts, diode Z3 breaks down. If the potential during the .positive half period exceeds 8.2 volts, above ground the diode Z8 breaks down. In the standby condition of the apparatus when the boundary is not being penetrated the potential impressed from Q2 on DR is of insuiiicient magnitude to cause the diodes Z3 and Z8 to break clown.

In the standby condition of the apparatus, when there is no intrusion at the boundary the only forces being impressed o-n the Detector Unit are those derived from remote sounds such as trafic or seismic disturbances. These sounds produce compressive waves of the fluid in the tubes 31 and 33 but the compressive waves Iare simultaneous and of equal amplitude in the two tubes because of the remoteness of the sources of the sounds. Under these circumstances the only signal at the output of LO is the 60-cycle interrogating signal. This signal lis impressed on the base of Q1 but, because of the clilter F1, not on the base of Q5. Q2 then delivers a flat-topped alternating-current signal to DR then selector lswitch CW and a like signal to the Ibase of Q6. The signal on DR is insufficient to break down thedio-des Z3 or Z8. The signal on Q6 produces a limited alternating-current signal which is impressed through C10 toy charge capacitor C20 through voltage-doubler diodes D and D21. The potential on C20 maintains Q4 conducting, maintaining relay R actuated and contact Ra closed to indicate that the transducers 53 and 55 are in proper operating condition.

On intrusion in the boundary, a signal is impressed on Q5 through the resistor R8 and the filter F1. Potentials of alternate polarity but usually of low frequency then appear at the' movable arm 104 of the switch CW through the resistor R9. Depending on the polarity of the potential impressed through R8, Z3 or Z8 is rendered conducting. The 60 cycle alternating-potential impressed on the base of Q6 is then suppressed and Q6 no longer passes the 60-cycle signal. Capacitor C20 then discharges through resistor R20 and diodes D22 and D23 and after a short time interval Q4 becomes non-conducting and relay R drops out opening Ra and closing Rb and producing the alarm.

In considering maloperation lof the apparatus, it may be assumed that the transducers 53 and 55 are connected in parallel. Under such circumstances, if one transducer is short circuited a short circuit is in effect connected across theother transducer. The interrogator potential then disappears and the relay R is deenefrgized so that its back contact Rb closes providing an alarm indicating the deficiency of the apparatus. If one of the transducers 53 or 55 is open-circuited, an unbalance is produced vin the intrusion pickup transducer in the output. A remote `sound would then immediately actuate the alarm.

Intrusion of the boundary can be distinguished from a defect in any transducer by the operation of relay R. As an'object moves through the boundary or towards and away from the boundary'the relay R is repeatedly energized and deenergized.'Provisions can be marde in the alarm indicator for energizing a characteristic alarm responsive to repeated operation of the contacts Ra and Rb to characterize an intrusion. A defect in a transducer is continuous so that the relay R remains continuously actuated. n

FIGS. 12 `and 13 show the response of the Detector Unit in the actual use of this invention. In producing FIG. 12 the response was observed for a person walking as disclosed in FIG. 12a. The tubes 31 and 33 are spaced a distance of about 2 to 5 feet. The person followed a path as shown by the broken lines in FIG. 12a. The person walked along the tube 33 a distance of one foot from tube 33, he then reversed his direction and moved along tube 33 -a distance 2 feet from 33, and then again reversed his direction and walked parallel to tube 33 a distance 3 feet from tube 33, then again reversed his direction and walked a distance 4 feet from tube 33 and nally reversed his direction and walked parallel to 33 a distance 5 feet from 33. The corresponding signals are presented in FIG. 12. It is seen that at one foot from 33 a signal of substantial amplitude is produced and as the distance from the tube 33 increases the amplitude of the signal is reduced until a relatively low signal is produced with the person walking parallel to 33, 5 feet from tube 33. As shown in FIG. l2, signals at more remote distances than 5 feet from tube 33 are balanced out by the respective phasing of the transducers 53 and 55. FIG. 13 shows the response of the transducers to a person crossing the fluid-filled tubes. The person crossed the tube 33 first and then the tube 31. It is seen that two wave trains are produced, one as the person crossed tube 33 and the other as the person crossed tube 31. The wave trains are of appreciable magnitude and would readily trigger the alarm indicator.

So that the invention may be more thoroughly understood, the following summary is presented:

The Detector Unit in its simplest form consists of two compliant hoses or tubes coupled to pressure-sensing transducers 53 and 55. The electrical outputs of the transducers 53 and 55 `are combined in such ya manner that the differential signal which they produce is capable of triggering the Alarm Indicator. The output signals of the transducers 53 and 55 are functions of pressure rate-ofchange where the low frequency cutoff is approximately .0l c.p.s. so that pressure variations resulting from earth shrinkage, temperature variations, and the like are not detected. `When an object enters the field of the Detector Unit over Section A (FIG. 3) an output of a positive polarity appears Iat the terminal 107 of the transducers 53 and 55. An object entering Section B produces an output of negative polarity at the terminal 107. A mechanical distunbance set up at some distance from both of these fields tends to cancel out because it excites both transducers 53 and 55 approximately the same. In practice, the transducers do not have equal sensitivities, and balancing may be effected with the capacitors C1 andCZ (FIGS. 10 and 1l).

An object entering the field of the Detector Unit compresses the soil and sets up a stress field within the nearest tube 31 or 33. The resulting strain field, even though almost infinitesimal at the tube, shrinks its diameter thus increasing the fluid pressure. This change in pressure is then sensed by the transducers.

FIG. 6 is the electrical network equivalent to vthe mechanical system of one transducerand tube. The following mechanical to electrical analogs are employed in its derivation:

Force, newtons- Voltage, volts Velocity, meters/ second Current, amperes Displacement, meters- Charge, coulombs Mass, kilogramselnductance, `henrys Compliance, meters/ kilogram Capacitance, farads Mechanical resistance, mks. units Resistance, ohms.

- A noise source N hasa pressurewvariation of force variation (product of pressure and area) at some point along the line. The pressure at the point would be the resultP of some force being. applied at the surface and transmitted to the tube by the impedance characteristics of the earth. The representation of this situation is a force source and its associated mechanical impedance is zn. For the general case the disturbing force accelerates the coupling fiuid, creates dissipation losses by the viscous shear of the fluid, and expands the tube in other regions because the earth and tube are compliant members. When the force reacts on the electromechanical transformer of the transducer, a voltage appears at the electrical terminals.

In the general case, the tube has a propagation veloc-` ity, c, determined approximately by the following equation when the uid viscosity can be neglected Mass of fluid per unit length bulk compliance of tube per unit', length At very low frequencies, the inertial effects of the fluid may be neglected and the equivalent circuit reduces to that shown on FIG. 7. For low frequencies, it is desirable to employ a low viscosity fluid since the series resistance shown is directly proportional to the dynamic viscosity of the fluid.

The labelled components in FIGURES 6 and 7 have the following significance.

The reference-force generator F shown on the far right in FIGS. 6 and 7 is a low-frequency electromechanical driver which transmits a signal to `the receiving pressure transducer for the purpose of making the system failsafe. The frequency of this signal (60 c.p.s.) is far removed from the expected alarm signal frequency (.01 c.p.s. to l c.p.s.). Failure to receive it constitutes a system failure, with a resulting continuous alarm or other indication. The alarm set off by intrusion would now be continuous and the operator would be able to distinguish between a system failure and a genuine alarm.

The electromechanical transducer housing shown in FIGS. 4 and 5 contains two fluid compartments 79 and 81 which couple the Huid 35 respectively to a pair of diaphragms 83. On each diaphragm 83 a piezo-electric disk 91 is bonded suchV that the output polarity of one disk 9.1 is'positive for a positive applied pressure and the other disk 9.1 negative for a positive applied pressure. Thus if equal pressure is applied to the fluid 35 each tube 31,and 33, the output potentials are equal and opposite so that for parallel electrically connected elements the output signal is zero. The output is a function of the pressure differential within the tubes, and localized sensitivity characteristic is obtained.

One of the unique features of the apparatus according to this invention is that it is insensitive to any accelera- Y tional motions in the three principal directions. FIG. 8a

shows the output signals for a given situation in which a positive pressure is applied to a tube. FIGS. 8b and 8c show the output response for an acceleration applied normally to the diaphragms. Acceleration response to acceleration in directions in the plane of the diaphragms 83 is negligible because in this case the ceramic disks 81 are in shear and these elements are insensitive to shear for the electrode configurations shown. The configurations shown in FIGS. 8b and 8c require that the tubes be connected to the same side and be in the same plane for differential operation and inertial balancing.

The arrangement shown in FIGS. 8a, 8b, and 8c is necessary for obtaining both pressure differential operation and inertial balancing. The inertial balancing reduces the direct pickup from the earth and is highly desirable because it cannot be eliminated by the balancing feature of the array.

Typically, the sensor elements or transducers may be connected in parallel. In this case, external balancing of the elements is achieved as shown in FIG. 10. The ele-` ments can be operated in series provided suitable insulation is employed between the metalic diaphragm and piezo-ceramic disk. Balancing for this connection is shown in FIG. 11.

The electrical signal from the transducer element is amplified by a Preamplifier and for test purposes may be applied to a Sanborn recorder having an alarm cir-cuit.

FIGS. l2 and 13 show recordings obtained on a Sanborn chart recorder with the transducers described, the tubes being buried at a depth of half a foot and five feet apart. The recording in FIG. l2 shows the recording obtained for normal walking parallel to one of the tubes 33, at various distances. The weight of the person was lbs. The spacing between the horizontal lines on the chart corresponds to a transducer voltage of 500 microvolts.

The recording in FIG. 13 shows an attempt to sneak across the tubes 31 and 33. Special effort was made not to step directly upon either tube, yet a significant signal was generated. The calibration was the same for each recording. It is probable that a low frequency cut-off (or a high pass filter) could be used to eliminate the very low. frequency drift shown on the recordings without destroying useful information.

The invention has the following advantages:

l) The apparatus embodying the invention is of relatively low cost; the cost is less than the cost of a microwave or like apparatus.

(2) The Detector Unit, being completely below ground, is completely out of sight, an important consideration in the application 0f the system to buildings within a city, where visible structures or objects would lend themselves to tampering and might be objected to.

(3) With the Detector Unit below ground, the apparatus is immune to damage from rain, wind, lawnmowers, and the like.

(4) The Detector Unit laas a strip sensitivity instead of area sensitivity as in the case of a single seismic sensor,

t 1 1 making the decribed apparatus especially suitable for perimeter guarding.

(5) By reason of the cancelling effect of the tWo tubes 31 and 33 for pressure signals originating from a distance, the described apparatus is relatively immune to noise such as might be encountered if a streetor sidewalk runs nearby to the guarded enclosure.

(6) The apparatus, being sensitive for several feet each side of each tube 31 and 33, is not subject to disabling without first triggering the alarm. Draining of the tubes or removal of electric power also triggers the alarm.

We cl-aim as our invention: v

1. A concealed apparatus for detecting penetration of an intruder across the boundary of a predetermined region comprising an elongated liquid-filled flexible tube means extending along said boundary and disposed below ground level, said liquid-filled flexible tube means` capable of transmission of forces of frequency from .0l to 10 c.p.s. over several hundred feet without significant attenuation, piezo-electric means responsive to the movement of said liquid in said tube means caused by the forces impressed on the wall of said tube by reason 'of intrusion of said boundary for converting said forces impressed at any point along the length of said elongated flexible tube into an electrical manifestation, and means responsive to said manifestation for producing an indication.

2. A concealed 4apparatus for detecting penetration of a boundary by an intruder of a predetermined region comprising flexible elongated tube means extending along said boundary, disposed below ground level and having a liquid therein including a freezing-temperature depressant, said liquid-filled flexible tube means capable of transmission of forces of frequency from .01 to 10 c.p.s. over several hundred feet without significant attenuation, transducer means responsive to the movement of said liquid in said tube means caused by the forces impressed on said tube means at any point along the length of said tube by intrusion of said boundary for converting said forces into an electrical manifestation, and means responsive to said manifestation for producing an indication.

3. A concealed -apparatus for detecting penetration of a boundary by an intruder of a predetermined region comprising liquid-filled tube means extending along said boundary, transducer means at each end of said tube means, alarm indicating means for signalling intrusion of said boundary connected to the transducer means at one end of said tube means, and interrogating signal means having a frequency of 60 cycles or less connected to the transducer means at the other end of said tube means for actuating said last-named transducer means to transmit an interrogating signal along said tube means to said transducer means at said one end of said tube means, said alarm indicating means responding to said interrogating signal to indicate that said transducer means at said one end of said tube means is in operating condition.

4. Apparatus for detecting penetration of a boundary of a predetermined region comprising first liquid-filled tube means, second fluid-filled tube means, said first and second tube means being disposed along said boundary as near as practicable to each other but sufficiently spaced to respond differentially to pressures near said first and second tube means, transducer means for converting pressure impulses along the liquid in said tube means into electrical manifestations, means connecting said first and second tube means to said transducer means so that pressure impulses of the same polarity in said tube means produces electrical manifestations respectively of opposite polarity in said transducer means, and means responsive to a difference in the magnitudes of said electrical manifestations.

5. Apparatus for detecting penetration of a boundary of a region including liquid-filled tube means extending along said boundary, a flexible diaphragm having a piezoelectric crystal mounted thereon connected to said tube means to be subjected to stress by the forces exerted by the liquid in said tube means when said fluid is displaced, said stress causing deflection both of said diaphragm and of said crystal, the relative thicknesses of said diaphragm and crystal being such that the neutral stress plane of the unit including said diaphragm and crystal is external to said crystal, and -alarm signaling means connected to said crystal responsive to the deflection thereof.

6. Apparatus for detecting penetration of a boundary of a predetermined region comprising first and second liquid-filled tube means disposed along said boundary, a first transducer connected to one end ofsaid first tube means, a second trandsucer connected to one end of said second tube means, alarm indicating means connected to said first and second transducer so as to respond only to a difference in the magnitudes of impulses substantially simultaneously impressed on said first and second tube means by reason of intrusion of said boundary, a third transducer connected to the opposite end of said first tube means, a fourth transducer connected to the opposite end of said second tube means, interrogating signal means connected to said third and fourth transducers for actuating said third and fourth transducers to transmit interrogating signals through said first and second tube means and monitor means connected to said first and second transducers responsive to said interrogating signals for indicating operative or non-operative response of each of said first and second transducers to said interrogating signals.

7. Apparatus for detecting penetration of a boundary t of a predetermined region comprising first and secondl transducer means.

liquid-filled tube means disposed along said boundary, first transducer means connected in counteracting-fluidimpulse receiving relationship yto the ends of one side of said first and second tube means, alarm signaling meansA connected to said first transducer means to produce an alarm indication -on the receipt by said first transducer means of unequal impulses from said first and second tube means, second transducer means connected to the ends on the-opposite side of said first and second tube means, and interrogating signal generating means connected to actuate said second transducer means to tranS-.

' impulse receiving relationship to the ends of said first and second tube means, alarm signaling means connected t0 said first transducer means to produce an alarm indication on the receipt by said first transducer means of unequal impulses from said first and second tube means, second transducer means connected to said first and second tube means, and interrogating signal generating means connected to actuate said second transducer means transmit through said tube means an interrogating signal received by said first transducer means in additive relationship thereby to test the operativeness of said first transducer means.

9. Apparatus for detecting penetration of a boundary of a predetermined region comprising first and second liquid-cooled tube means disposed along said boundary, first transducer means connected in counteracting-liquidimpulse receiving relationship to the ends of said first and second tube means, and alarm signaling means connected to said first transducer means to produce an alarm indication on the receipt by said first transducer means of unequal impulses from said first and second tubey means.

References Cited UNITED STATES PATENTS 640,273 1/1900 Coleman. 1,915,167 6/1933 Salsbury.

(Other references on following page) 14 3,134,970 5/1964 Kelly et al 340-261 3,135,951 -6/1964 Byrne 340-258 3,258,762 6/ 1966 Donner 340-261 JOHN W. CALDWELL, Primary Examiner. D. L- TRAFTON, Assistant Examiner.

U.S. C1. X.R. 

